Ionization gauges, more specifically Bayard-Alpert (BA) ionization gauges, are the most common non-magnetic means of measuring very low pressures. The gauges have been widely used worldwide. These gauges were disclosed in 1952 in U.S. Pat. No. 2,605,431, which is herein incorporated by reference in its entirety. A typical ionization gauge includes an electron source, an anode, and an ion collector electrode. For the BA ionization gauge, the electron source is located outside of an ionization space or anode volume which is defined by a cylindrical anode screen. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source to and through the anode, cycle back and forth through the anode, and are consequently retained within, or nearby to, the anode.
In their travel, the electrons collide with molecules and atoms of gas that constitute the atmosphere whose pressure is desired to be measured. This contact between the electrons and the gas creates ions. The ions are attracted to the ion collector electrode, which is typically connected to ground. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (Iion/Ielectron), where S is a coefficient with the units of 1/Torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range.
The operational lifetime of a typical ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges and electron sources (cathodes) fail in minutes or hours when operated at too high a pressure or during operation in gas types that degrade the emission characteristics of the electron source. Sputtering is a problem when operating the ionization gauge at high pressures, such as above 10−4 Torr. This is a problem at high pressure because there is more gas to ionize. This sputtering is caused by high energy impacts between ions and components of the ionization gauge. Ions with a high energy may impact a tungsten material that forms a collector post of the ionization gauge. These impacts result in atoms being ejected from the collector post and envelope surfaces with significant internal kinetic energies. Ejected material can travel freely to other surfaces within the line of sight of the sputtered surfaces, and can cause gauge failure by coating the cathode or by coating of the feed-through insulators of the gauge, which can result in electric leakages.
Therefore, there is a need for an ionization gauge design that minimizes or eliminates the above mentioned problems.